Respect for the police is a must and rude talk or shouting should be avoided in every case, guilty or not guilty. Thai police officers have much greater powers on action than for example their European counterparts and will not accept disrespectful behavior. Common offences such as not wearing a helmet will get you a 500 Baht fine and a 2 hour wait at the local police station to pay your fine. Parking on the wrong side of the road will get your bike or car chained up and another long wait to pay your fine. If you rent a motorcycle and you get involved in an accident you will have to pay for all the damage regardless if you are in the right or in the wrong. Basically that's the way it is, motorcycles are not insured and the Thai police will take the side of their fellow countrymen over you in any dispute. Having said that, the tourist police in Koh Samui are also aware of the little tricks that the rip-off merchants use (such as short changing at money exchange booths) so if you feel that you have been done an injustice then stand on your ground, remain cool at all times whilst awaiting their arrival.